Mobile service and drilling rigs have been commonplace for many years and are primarily used to drill boreholes and to perform various other downhole operations. In a great number of applications, the total weight of the shaft and the rigging equipment can far exceed the desirable weight at the drill head for optimal drilling and other operations. As shaft equipment extends deeper into a downhole, additional shaft or tubular sections must added to increase the length of the apparatus. As such a process continues, the gross weight of the shaft and rigging equipment increases and naturally exerts additional force on the drill head. In some applications, this weight may increase by an average of over six pounds for every vertical foot of shaft. If the total weight of the shaft and rigging equipment is continuously allowed to be placed on the shaft head, undesirable results may occur. For example, excessive weight on a drill bit during drilling can result in a bore hole which is not straight.
To eliminate problems due to excessive shaft weight, weight indicators have been developed and used since the early 20th Century. By using a weight indicator, a rig operator can observe the relative weight of the drilling equipment that is being supported by the equipment rather than at the shaft head. Using this information, the operator can either increase or decrease the tension in the rigging to vary the net weight placed on the drill head. Because of their usefulness, weight indicators have become an essential tool for many downhole operations and have become ubiquitous in the downhole drilling and servicing industry.
There are generally two types of weight indicators which have become the most prevalent in the industry: the diaphragm type weight indicator and the pad type weight indicator. Both types use hydraulic fluid in a closed loop system in conjunction with analog bourdon tube type dial gages to indicate a change in pressure in the system. Each type also has changeable load dials having varying scales to correspond to different pad diameters or rigging systems which may be used in a given application. Another similarity is that both types of indicators require that the fluid pressure be manually dampened prior to engaging in certain operations which can lead to sharp changes in system pressure. This is required to protect the bourdon tube type gauges that are typically used for these indicators. Even though these and other similarities exist, the indicators are very different with respect to the location on the rig where the measurements are being taken and the type of sensing equipment utilized. A discussion of each type of weight indicator follows.
The diaphragm type weight indicator generally has a diaphragm unit, a gauge and a hose connecting the diaphragm unit to the gauge. This type of system is generally pre-filled with fluid and the fluid level is not readily changeable by the end user. In practice, the diaphragm unit is clamped onto the static deadline of a rig. When installed, the diaphragm forces a one inch deflection in the line. As the tension in the deadline increases from additional loads on the rigging, the deadline tends to straighten against the diaphragm. As this straightening force increases, the diaphragm is compressed thereby causing an increase in fluid pressure which is reflected at the gauge. The hook load, which is generally supported by a 2, 4 or 6 line tackle system, is directly proportional to the tension in the deadline. To accommodate the varying tackle arrangements and line numbers, differently scaled load dials are used to indicate an actual weight measurement. In general, the fluid pressure will range from 0 pounds per square inch at no load to 100 pounds per square inch at full load for this type of system.
The diaphragm weight indicator performs well at sensing differences in the hook load, but is generally not capable of providing a measurement for absolute hook load. Several factors contribute to this condition. First, because the hydraulic fluid is in a closed system, the pressure in the system is dependent on the fluid temperature. Thus, large ambient temperature changes will result in different readings for the same hook load. Second, the fluid level in the system is not easily checked or serviced by the user. Thus, it is not always known if the fluid levels are at the appropriate level. Third, excessive frictional forces in the rigging tackle system can cause readings to become distorted. Fourth, and lastly, the connection of the diaphragm to the deadline and the resulting contact points are not always consistent. This is especially true if system components become worn over time. Although all of these factors do present problems with determining an absolute load, the system does provide generally reliable information for changes in hook load. Further, only knowing load changes is generally sufficient for most service and drilling operations. As a result, the diaphragm type weight indicator has become very popular over a long period of time as a rugged and reliable tool for downhole operations.
The pad type weight indicator relies upon a very different method of operation and can be used to obtain an absolute weight value for the hook load. It should be noted that when a pad type indicator is being used, an operator would not also use a diaphragm type weight indicator at the same time. Rather, the operator would choose one type of indicator or another based on the particular application at hand. The pad type indicator relies upon two load cells, each of which is located under the service rig's jack screws or mast legs. The load cells are generally 6, 7 or 8 inches in diameter and are connected via hoses to individual bourdon type pressure indicating gauges and also to a pressure integrator or summarizer configured to add the two pressures together. The pressure integrator has a tube connected to a third gauge which indicates the total system weight. The fluid operating pressure in this type of system is typically ranges from 0 to 2,000 pounds per square inch (psi) and potentially up to 3,000 psi. Additionally, the load dials for this type of system are differently scaled based on the pad diameter and are generally able to rotate such that the load can be zeroed or tared. This feature allows the user to compensate for the weight of the mast itself, forces exerted by the guy wires stabilizing the mast and potential temperature effects on the closed system. Because the pads are directly under the mast legs, the indicated load is not skewed by frictional forces in the rigging system. There is not a concern with inaccuracies due to fluid level for this system because the user can readily monitor and ensure proper fluid levels. As such, the pad type weight indicator provides a reliable and rugged option to the diaphragm type indicator and is also capable of providing a good indication of absolute hook load.
Even though the diaphragm and pad type weight indicators have gained wide acceptance in the well service industry and are considered vital tools, improvements are possible and desired. This is especially true in light of the many technological advances that have recently occurred with respect to the electronics industry.